The term déjàvu is French and means, literally, "already seen." Those who have experienced the feeling describe it as an overwhelming sense of familiarity with something that shouldn't be familiar at all. Say, for example, you are traveling to England for the first time. You are touring a cathedral, and suddenly it seems as if you have been in that very spot before. Or maybe you are having dinner with a group of friends, discussing some current political topic, and you have the feeling that you've already experienced this very thing -- same friends, same dinner, same topic.

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The phenomenon is rather complex, and there are many different theories as to why déjà vu happens. Swiss scholar Arthur Funkhouser suggests that there are several "déjà experiences" and asserts that in order to better study the phenomenon, the nuances between the experiences need to be noted. In the examples mentioned above, Funkhouser would describe the first incidence as déjàvisite ("already visited") and the second as déjàvecu ("already experienced or lived through").

As much as 70 percent of the population reports having experienced some form of déjà vu. A higher number of incidents occurs in people 15 to 25 years old than in any other age group.

Déjà vu has been firmly associated with temporal-lobe epilepsy. Reportedly, déjà vu can occur just prior to a temporal-lobe seizure. People suffering a seizure of this kind can experience déjà vu during the actual seizure activity or in the moments between convulsions.

Since déjà vu occurs in individuals with and without a medical condition, there is much speculation as to how and why this phenomenon happens. Several psychoanalysts attribute déjà vu to simple fantasy or wish fulfillment, while some psychiatrists ascribe it to a mismatching in the brain that causes the brain to mistake the present for the past. Many parapsychologists believe it is related to a past-life experience. Obviously, there is more investigation to be done.